When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear” or a “Cartesian” scan. There are many other k-space sampling patterns used by MRI systems. These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space.
An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1 DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection.
An important application of MRI is angiography. The current gold standard for evaluating the arterial system is digital subtraction angiography (DSA), which provides detailed images that can be used to determine the presence and extent of conditions such as arterial stenoses and occlusions. DSA has a high specificity and sensitivity, but despite advances in DSA and catheter technology, risks associated with angiography persist and include allergic reaction, reduced renal function, and complications related to the arteriotomy and intravascular catheter manipulation. Because of its invasiveness and risk, it is unsuitable as a routine screening test for vascular disease.
Magnetic resonance angiography (MRA) has great potential as a non-invasive alternative to catheter-based DSA, as it is safer and generally less costly. MRA uses the magnetic resonance phenomenon to produce images of the human vasculature. A paramagnetic contrast agent such as gadolinium can be injected into the patient prior to the MRA scan to enhance the diagnostic capability of MRA. By infusing a sufficient quantity of gadolinium contrast agent into the blood, the T1 relaxation time can be shortened to 100 ms or less, particularly during the first pass of contrast agent through the arteries. Under these conditions, a three-dimensional spoiled gradient-echo pulse sequence is used to acquire data, typically using a repetition time (TR) as short as 3-6 ms and echo time (TE) as short as 1-2 ms. If the number of phase-encoding lines are chosen appropriately, then the data can be acquired within a reasonably short period, for example, on the order of 15-30 s. The short acquisition is important because it ensures that most data is acquired during the peak arterial phase of contrast enhancement.
If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the renal arteries or carotid, the separation between arterial and venous enhancement can be as short as 6 seconds. Thus, many MRA studies can be difficult to implement without including some degree of unwanted venous signal. The short separation time between arterial and venous enhancement also dictates the use of acquisition sequences having either a low spatial resolution or very short repetition times (TR). Short TR acquisition sequences severely limit the signal-to-noise ratio (SNR) of the acquired images relative to those exams in which longer TRs are possible. The rapid acquisitions required by first pass CEMRA methods thus impose an upper limit on either spatial or temporal resolution.
Contrast-enhanced MRA of the peripheral arteries has become a routinely used imaging study in recent years. It depicts arterial stenoses and occlusions and aids treatment planning of patients with known or suspected peripheral arterial occlusive disease. Currently, a widely used approach involves the intravenous infusion of a gadolinium contrast medium, combined with use of a stepping table and acquisition of manually-positioned coronal scan volumes at several table positions (“stations”).
Although the paramagnetic contrast agents in clinical use have excellent safety profiles, the recent discovery of nephrogenic system fibrosis (NSF) as a potential side effect of gadolinium administration has dampened enthusiasm for contrast-enhanced MRA and led to a “black box” warning from the United States Food and Drug Administration. The risk of NSF relates to the presence of severely impaired renal function, as well as to the dose and stability of the contrast agent. Unfortunately, peripheral MRA studies typically entail the administration of high doses of contrast agent. Given the risk of NSF as well as the substantial cost for the high dose of contrast agent, a non-contrast alternative would be beneficial.
In addition to contrast-enhanced MRA, several contrast mechanisms have been used to create angiograms that are not dependent on contrast administration. These include time of flight (TOF), phase contrast (PC), subtraction of images acquired at different phases of the cardiac cycle (as originally reported and more recently using the fresh blood imaging technique), using inversion recovery and T2-preparation for background suppression, and contrast based on the bright blood signal produced by balanced steady-state free precession (bSSFP) pulse sequences. Each of these non-contrast-agent-based imaging techniques has drawbacks or is not suitable for particular imaging studies. For example, early work with TOF and PC techniques demonstrated that it was feasible to depict intra- and extracranial vascular lesions of the head and neck with an accuracy approaching that of conventional x-ray angiography. Phase contrast techniques also enable functional evaluation through the measurement of blood flow. However, time-of-flight and phase contrast methods are inadequate for major body MRA applications, such as imaging of the renal or peripheral arteries.
In addition, all of these imaging techniques are susceptible to motion artifacts. This is especially true for image subtraction techniques, where even motion at a sub-pixel level can cause subtraction artifacts that degrade diagnostic quality. For example, in contrast-enhanced breast MRI, subtraction artifacts routinely impede the detection of small enhancing tumors and can lead to false positive diagnoses. Throughout the history of clinical MR, an overriding goal has been to eliminate artifacts resulting from motion, which degrade the images and can render them uninterpretable. In the case of MRA, a particular focus has been on the suppression of periodic image artifacts, called “ghosts,” which result from non-steady flow patterns in arteries.
It would therefore be desirable to have a system and method for performing background-suppressed 3D MRI without the need for image subtraction that provides a high signal-to-noise ratio (SNR), does not suffer from artifacts, and is versatile.